Generating pure hydrogen at an elevated temperature by reacting a fossil fuel with steam and simultaneously permeating hydrogen through a hydrogen selective membrane is well known in the art, as reviewed for example, in U.S. Pat. No. 6,171,574 B1 (2001), of common assignee, incorporated herein by reference. Reference is further made to U.S. Pat. Nos. 5,326,550 (1994) and 6,3311,283 B1 (2001), which patents describe such membrane reactors in which natural gas is steam-reformed in the presence of a fluidized catalyst bed.
Of particular relevance to the present invention is the experimentally verified Sievert's and Fick's Laws of the prior art, according to which the hydrogen flux through the membrane is proportional to the difference of the square roots of the hydrogen pressures in the high-pressure and low-pressure chambers of a palladium-bearing membrane generator of pure hydrogen. To lower the pure hydrogen pressure, a sweep gas (e.g., nitrogen or steam) has been used in the art to enhance hydrogen flux. This technique therefore increases not only the hydrogen flux, but can also increase the yield of pure hydrogen from an impure hydrogen-containing gas mixture by lowering the hydrogen partial pressure required at the impure stream exit to maintain hydrogen flux through the membrane. However, these benefits are attained at the expense of having to separate the sweep gas from the hydrogen product and having to pressurize the pure hydrogen product. Using more sweep gas per unit of hydrogen permeated evidently provides greater benefits but at a greater expense for the subsequent separation/pressurization.
The capability of permeating hydrogen in situ from a fossil fuel reforming reactor through an anodic hydrogen selective palladium-alloy membrane in a molten alkaline hydroxide electrolyte fuel cell has been described, for example, in U.S. Pat. Nos. 3,407,049, 3,407,094 (1968) and 3,669,750 (1972). (This fuel cell was abandoned due to the costly removal of carbon dioxide from large quantities of cathodic air.)
The relevance of these patents to the pressurizer of the present invention is illustrated by the following plausible interpretation. Under D.C., power a permeated hydrogen atom reacts at the anodic interface with, for example, one hydroxyl ion of a molten alkali hydroxide, producing one water molecule and one electron. Due to this practically instantaneous reaction, the effective hydrogen gas pressure on the membrane's electrolyte-facing side is zero, and the hydrogen flux through the membrane is therefore maximized in accordance with Sievert's Law. In the fuel cell case, the liberated electron ionizes corresponding amounts of the air's oxygen and water at the cathode thus generating the D.C. power in the external circuit and replenishing the hydroxyl ion in the electrolyte, all at ambient pressure. To the contrary, the pump of this invention requires a D.C. power supply and a pressurized chamber and, advantageously no moving parts.
Moreover, the cathodic evolution of hydrogen is now subject to an overvoltage the magnitude of which depends not only on current density and temperature, but significantly on the nature of the cathode. This is shown, for example, in the article entitled “Ni—Mo—O alloy cathodes for hydrogen evolution on hot concentrated NaOH solution” by A. Kawashima, T. Sakaki, H. Habazaki, K. Hashimoto, Material Science and Engineering A267 (1999) 246–254 incorporated herein by reference. In particular, FIG. 8, page 251, illustrates the sharp lowering of the overvoltage by the Ni—Mo—O coating of nickel vs. uncoated nickel. By way of another example, a publication, also incorporated herein by reference, entitled “Electrode properties of amorphous nickel-iron-molybdenum alloy as a hydrogen electrocatalyst in alkaline solution”, by W. Hu, Y. Zhang, D. Song, Z. Zhou, Y. Wang, Materials Chemistry and Physics 41 (1995) 141–145, details an electrolytic technique of coating, for example, a copper or nickel sheet with a Ni/Fe/Mo alloy resulting in effective and stable hydrogen-evolving cathodes in the electrolysis of 30 wt. % KOH at 70° C.
Electrochemical hydrogen compression is well known in the art for many years. Reference is made to recent development of compressors using PEMs as the electrolyte, as described for example in two publications, one entitled “Electrochemical hydrogen compressor” by B. Rohland, K. Eberle, R. Ströbel, J. Scholta and J. Garche, Electrochimica Acta, Vol. 43, No. 24, pp. 3841–3846 (1988) and the other entitled “The compression of hydrogen in an electrochemical cell based on a PE fuel cell design”, by R. Strobel, M Oszcipok, M. Fasil, B. Rohland, L. Jorissen, and J. Garche, J. of Power Sources, Vol. 105, pp. 208–215 (2002).
The use of a phosphoric acid electrolyte in an electrochemical hydrogen purifier of impure hydrogen, including compression thereof, is described in U.S. Pat. No. 4,620,914 (1986). Here porous gas diffusion electrodes were employed. Further the generation of electrolytic hydrogen utilizing palladium or palladium alloy electrodes is described in U.S. Pat. No. 4,078,985 (1974) in which the electrolyte is an aqueous solution containing 20% of NaOH (Col. 3, 1 3–4).
Electrochemical hydrogen pumping using ceramic proton conductors at 900° C. has recently been described in publications by H. Matsumoto, F. Asakura, K. Takeuchi and H. Iwahara, Solid State Ionics, [129 (2000) pp 209–218]; by H. Matsumoto, Y. Iida and H. Iwahara [ibid. 127 (2000) pp 345–349)]; and by H. Iwahara [ibid. 125 (2000) pp 271–278]. Here hydrogen is electrochemically pressurized by applying a D.C. potential across a SrCeO3-based, proton conducting electrolyte/membrane. In this scheme, hydrogen molecules are dissociatively ionized into two (2) protons and two (2) electrons on one face of the electrolyte/membrane and the hydrogen molecule is reassociated with the two electrons on the other face of the electrolyte/membrane, the higher pressure being achieved by increasing the applied voltage. This electrolyte/membrane is practically limited to excessively high temperatures due to the very low protonic conductivity of the membrane.